1. Field of the Invention
The present invention relates to a digital still camera having an image blur correcting function.
2. Description of Related Art
In a silver-salt film camera, its image-taking sensitivity (ISO sensitivity) cannot be changed unless a loaded film is replaced, however, in a digital still camera, its image-taking sensitivity can be easily changed by changing the gain of signals from an image pickup device such as a CCD.
Therefore, an image-taking sensitivity suitable for image-taking conditions can be set by a camera operator or at the camera side. For example, when it is desired to reduce affects from image blur by increasing the shutter speed, the image-taking sensitivity is set to be higher in accordance with the shutter speed increase.
Furthermore, as an image blur prevention system, a system which optically prevents image blur by moving a part of the lenses of the image-taking optical system has been proposed.
Herein, an image blur prevention system is briefly described. Camera shake when taking an image is generally vibrations with frequencies of 1 to 12 Hz, and at the point of shutter releasing, in order to make it possible,to take a photograph without image blur even when such camera shake occurs, it is necessary that the abovementioned camera shake due to hand shake is detected and the correction lens is displaced in accordance with a detected value. Therefore, in order to make it possible to take a photograph without image blur even when camera shake occurs, first of all, accurate detection of camera shake becomes necessary, and second, a displacement of the optical axis of the camera due to the camera shake must be corrected.
This shake (camera shake) detection can be carried out, in principle, on the camera, by mounting a vibration sensor which detects the angular acceleration, the angular velocity, and angular displacement, etc., and a camera shake sensor which outputs angular displacement by electrically or mechanically integrating an output signal of this vibration sensor. Then, the correcting optical element which makes the image-taking optical axis eccentric is driven based on this detection information to carry out prevention of image blur.
A conventional camera vibration isolation system is proposed in Japanese Patent Application Laid-Open No. H9(1997)-43658. The outline of this is described with reference to FIG. 9. This vibration isolation system is a system which suppresses image blur caused by vertical shake 81p and horizontal shake 81y of the camera in the directions shown by the arrows 81, and is provided at the interchangeable lens of a single-lens reflex camera.
In the same figure, the reference numeral 82 denotes a lens barrel. Shake sensors 83p and 83y detect vertical shake and horizontal shake of the camera, respectively, and their shake detection directions are indicated by the reference numerals 84p and 84y, respectively.
The reference numeral 85 denotes a shake correcting optical unit. Coils 86p and 86y provide thrust for the shake correcting optical unit 85, position sensors 87p and 87y detect the position of the shake correcting optical unit 85. These coils 86p and 86y and position sensors 87p and 87y form a position control loop, and power is supplied to the coils 86p and 86y so that the positions detected by the position sensors 87p and 87y reach set target values based on the outputs of the shake sensors 83p and 83y to prevent image blur at the image plane 88.
However, in a digital still camera, if the shutter speed is increased to prevent image blur and the image-taking sensitivity is raised, noise is also amplified, resulting in lowering in the S/N ratio of the image-taking system and deterioration of the image.
Furthermore, when an image blur correcting system is provided to prevent image blur, the shutter speed is lowered, and the image-taking sensitivity is set to be lower than a standard image-taking sensitivity to improve the S/N ratio, signals from the image pickup device for an object portion having a certain level of higher brightness become saturated, and there is a possibility that the image of this object portion is taken to be white (over exposure).